This invention relates to a drill countersink assembly for accurately preparing precision holes in materials for fasteners, specially for aircraft fasteners used to attach aircraft skins to the frame of the aircraft or wing assembly where such fasteners need to fit precisely into the material to provide the optimum strength of the bonded structure and to avoid mismatches that would allow the head of the fastener to be either above or below the skin and thereby increase the drag on the surface of the plane. In particular, this invention is concerned with the ability to accurately match the precise form of the fastener head, matching the angle of the fastener head and fillet radius, so that when positioned in the hole, the fastener head is as perfectly matched to the hole in the material as formed by the drill countersink assembly and flush with the skin of the plane. In addition to increasing the precision of the hole formed by the invention, the invention reduces the costs of preparing such holes by providing an easy to use assembly with replaceable wear components and a reusable countersink body.
Prior to this invention, it was necessary to use a step drill with a fixed countersink construction in order to obtain a cutting tool with the necessary accuracy for preparing the desired match between the fastener and the recess created by the machining process.
Drill countersink assemblies are well known as means of drilling and countersinking a hole in a single operation. U.S. Pat. Nos. 5,915,895; 7,090,447; and 7,131,798 are recent examples of improvements to such assemblies. The designs mentioned in these patents as well as other prior art for drill countersink assemblies, however, do not provide for the necessary accuracy in the assembly to be useful for the purposes of this invention. Specifically, small alignment errors occur in the clamping of the drill into the countersink assembly that creates a slight run out at the tip of the drill relative to the shank of the countersink assembly. Such run out can enlarge the drilled hole creating an unacceptable mismatch between the hole and the fastener body.
A shorter drill countersink assembly that relies on the drill shank for the alignment of the drill point (such as disclosed in U.S. Pat. Nos. 5,071,295 and 7,147,409) avoids the misalignment errors at the drill point. But, these shorter drill countersink assemblies lack the accurate positioning of the countersink insert and the ease of creating that position, whereas that feature is inherent in the present invention.